Tissue ablation for the treatment of cardiac arrhythmias may use various sources or heat or cold to modify or prevent conduction within the tissue of the heart with a therapeutically beneficial aim of eliminating the cardiac arrhythmia. Cosman in U.S. Pat. No. 4,411,266 describes a radio frequency lesion electrode design with a thermocouple temperature sensor in its distal uninsulated tip. The instrument was described as a common instrument for neurosurgery to destroy tissue by heat. Because temperature is the basic lesioning or destruction parameter, temperature control or monitoring of the electrode's tip was an essential means for carefully grading the degree or destruction and quantifying the lesion size. A rapid and faithful readout of tissue temperature was often critical to safety and successful results. Thermistor sensors posed a limitation on the outer diameter shaft size of the catheter. Thermocouple sensors did not have the same limitation but presented difficult technical problems in fabrication and suitability in accuracy and speed of thermometric response for very small gauge radio frequency lesion electrodes. A temperature sensor not at the extreme tip end of the electrode produces various sources of inaccuracies. Because the sensor is placed internally in the tip, it senses only the average tissue temperature around the tip which may be significantly below that at the very tip. Such a situation can produce dangerous inaccuracies in a critical procedure. There is a temperature gradient due to the finite mass and heat conduction effects. Thus, the sensor, when not exactly at the surface of the end, will never be at the temperature of the hottest, most critical region near the very tip of the electrode. Langberg in U.S. Pat. No. 4,945,912 describes a catheter for ablating cardiac tissue with means to control the RF power applied to tissue surrounding the catheter tip. Lennox et al in U.S. Pat. No. 4,955,377 describe a device and method for heating tissue, the device having a catheter shaft for insertion into a patient's body, a thermistor sensor to control the application of the current and a carefully controlled therapy can be conducted at a constant temperature.
Cosman in U.S. Pat. No. 4,966,597 describes a device with a faithful and rapid temperature reading in the tissue. Because the electrical junction is exactly at the surface of the electrical surface means no thermal mass effects at the tip and the temperature is precisely the temperature of the adjacent tissue outside of the electrode. Lennox in U.S. Pat. No. 5,122,137 describes a catheter with a temperature sensor carried by and in a thermally conductive relationship with a thermally conductive electrode. The temperature sensor senses the temperature of the electrode, and thereby indirectly senses the temperature of tissue in contact with the electrode. The sensor is connected by a feedback line to a control circuit that automatically modulates RF power applied to the electrode.
Langberg, in U.S. Pat. No. 5,230,349 describes the temperature boundary between viable and non-viable tissue as approximately 48 degrees Celsius (C). Tissue heated to a temperature above 48 C is non viable. The objective of ablation is to elevate the basal tissue temperature, generally at 37 C, fairly uniformly to the ablation temperature above 48 C, keeping, however, the hottest tissue temperature below 100 C. At approximately 100 C, charring and tissue desiccation take place which seriously modifies the electrical conductivity of blood and tissue, and causes an increase in the overall electrical impedance of the electrical heating circuit and a drop in the power delivery to the tissue. Charring is particularly troublesome at the surface of the catheter electrode since the catheter must be removed and cleaned before the procedure can continue. The active electrode temperature is the result of the balance between conductive heating and convective cooling from the blood.
Edwards et al in U.S. Pat. No. 5,456,682 describe an ablation electrode with a temperature sensing element located on the energy emitting body of the ablation electrode. The temperature sensing element senses the temperature of the tissue being ablated by the electrode. The electrode includes a thermal insulating element located between the energy emitting body and the temperature sensing element. At least one, and preferably all, the temperature sensing elements are thermally insulated. The thermally insulated temperature sensing element measures true tissue temperature, without being affected by the surrounding thermal mass of the electrode.
Brucker et al in U.S. Pat. No. 5,500,012 describe an ablation system for treatment of tachyarrhythmia and identify several problems of the ablation of myocardial tissue as including blood coagulated onto the electrodes during ablation and sometimes difficult to know whether the tissue is being destroyed or whether the energy is being diverted to the catheter or the blood.
Panescu et al in U.S. Pat. No. 5,688,267 describe systems and methods including multiple temperature sensing elements. One element senses tissue temperature. A second element senses electrode temperature. The systems and methods control the supply of ablation energy to the electrode based, at least in part, upon the multiple temperatures sensed by the different temperature sensing elements.
Panescu et al in U.S. Pat. No. 5,735,846 describe systems and method for ablating body tissue using an electrode for contacting tissue at a tissue-electrode interface to transmit ablation energy at a determinable power level. The systems and methods employ a processing element to derive a prediction of the maximum tissue temperature condition occurring beneath the tissue-electrode interface. In one implementation, the processing element controls the power level of ablation energy transmitted by the electrode based, at least in part, upon the maximum tissue temperature prediction. In another preferred embodiment, the processing element samples the power level at which the electrode transmits ablation energy, the temperature of the electrode, and the rate at which heart is removed from the electrode to derive the maximum tissue temperature.
Chen in U.S. Pat. No. 5,849,028 describes an electrophysiology catheter suitable for radiofrequency ablation of cardiac tissue with multiple long electrodes and multiple temperature sensors in the proximity of the tissue contact sites and further comprising a close-loop temperature control mechanism for each electrode with at least a temperature sensor on an adjacent tiny ring. The securing point of the temperature sensor on the electrode is usually on the opposite side of the tissue contact point to avoid temperature surge when the RF energy is suddenly delivered. And the measured temperature from said sensor does not reflect the true real-time temperature at the tissue contact point for temperature control purpose. Chen describes as useless when the measured temperature does not reflect the true temperature. Chen provides an ablation catheter having a temperature sensor secured adjacent to an electrode, while not in contact with any electrode, to independently and accurately control the energy delivery to each electrode; wherein the temperature sensor is secured to the proximity of the tissue contact site.
Simpson et al in U.S. Pat. No. 6,049,737 describe a catheter having a plurality of electrodes arranged in a linear array, temperature sensors located at the electrodes and each shares a common lead with the power circuitry. The temperature sensor signal is received by a power control system during the off-period of the duty cycle of the particular electrode. In the case where a catheter has a band electrode, such as for the treatment of atrial fibrillation by the ablation of tissue, a single temperature sensor mounted to the band may not provide the temperature of the tissue contacting the band electrode. Typically, the side of the band which is in direct contact with the tissue becomes significantly hotter than the rest of the band electrode that is cooled by the blood flow. Thus, the temperature reading can be dramatically influenced by the rotational orientation of the catheter during RF ablation. If the band is oriented so that the single temperature sensor is not in contact with the tissue during the application of ablation energy, not only would there be a time lag in the sensor reaching the tissue temperature, but due to the effect of the cooling blood flow, the sensor reading may never approach the actual tissue temperature.
To overcome the effect that the rotation orientation of the band electrode has on temperature sensing, two thermocouples, positioned at different locations of the band electrode, may be used. A theory is that having a sensor in contact with tissue is more likely. While attachment of multiple temperature sensors to the band electrode can result in a higher probability of sensing the actual tissue interface temperature, this also increases the number of wires occupying space within the catheter. As is well appreciated by those skilled in the art, an increase in the number of internal wires could mean an undesirable increase in catheter diameter to accommodate those wires. Conventional types of thermocouples each require a thermocouple wire pair. Two thermocouples at each band electrode would result in four wires per band electrode so that the use of multiple temperature sensors may not be practical, particularly where the catheter carries multiple band electrodes that require temperature monitoring.
The larger the catheter, the more traumatic it is to the patient. Also, the more difficult it may be to negotiate the patient's vessels to position the catheter at the desired location in the heart. It is desirable to provide a catheter with as small a diameter as possible. A limiting factor in reducing the size of the catheter is the amount of devices and leads that must be carried inside the catheter. In the case of a catheter having ten band electrodes with two thermocouple temperature sensors at each electrode, a total of fifty wires would be necessary; one power wire for each electrode and two wires for each thermocouple. The size of fifty wires inside a catheter can be significant, causing an increased diameter of the catheter. Yet it is desirable to retain the electrodes and the associated temperature sensors so that more precise control over the energy applied to the biological tissue can be effected. Thus, it would be desirable to reduce the number of wires within a catheter, yet retain the same functionality.
As designs for ablation catheters incorporate a significant number of electrodes and shapes, it has become important to limit the number of wires that need to be threaded through the catheter and especially through the proximal band electrodes on the catheter. Further, by limiting the number of thermocouples that need to be placed in each electrode, the number of wires that need to be threaded can be achieved. The physician user often finds it difficult to manipulate the ablation catheter to the precise location to achieve the desired therapeutic effect. It is, therefore, desired that an ablation system automatically adapt to any catheter orientation and not require the user to rotate the catheter to a specific orientation.
The invention fulfills the needs described above and others.